This invention relates generally to aircraft nose landing gear assemblies, and, more particularly, to hydraulic steering actuators for aircraft nose landing gear assemblies.
Modern commercial aircraft require efficient nose landing gear steering actuators to provide steerage during taxiing of the aircraft. Various types of steering actuators have been employed for this purpose, yet certain undesirable features have characterized the steering actuators heretofore available.
For example, one type of steering actuator currently widely used on large commercial jet aircraft employs a pair of linear hydraulic push-pull actuators. Each push-pull actuator is fixed at one end with respect to the airplane frame and is connected at its other end to a radial extension of a rotatable landing gear shock absorber strut cylinder. The linear push-pull actuators are arranged approximately parallel to one another on opposite sides of the shock strut assembly. Steering of the nose wheel is effected by simultaneously pushing with one linear actuator and pulling with the other actuator. The system thus operates as a dual crank and crankshaft mechanism for translating the linear force of the actuators to rotational torque applied the landing gear shock strut. The disadvantage of this type of system is that torque is unevenly applied to the shock strut over its full range of rotation. As with any crankshaft mechanism for translating linear force to torsional force, the linear actuators are most efficient when the landing gear is at dead center and each actuator is at the top of its stroke. A sinusoidal relationship between the angular displacement of the landing gear and the torque applied by the actuators causes the actuators to become increasingly less efficient as they approach the bottom portions of their strokes and the landing gear is rotated away from dead center. Because of this inefficiency, the actuators must necessarily be designed to have excess power at the dead center position in order that they will have sufficient power to maintain the steering gear in a deflected position at high angles of rotation. As a result of this inefficiency and the design compensation necessary to overcome it, the steering actuators are relatively heavy and bulky. Furthermore, even when heavy-duty linear steering actuators are employed, they typically operate near their maximum power output when the landing gear is steered to a large steering angle. Because of the sinusoidal relationship between the torque applied to a crankshaft steering device and the angular displacement of the device, it follows that even heavy-duty linear steering actuators are capable of applying only a limited amount of torque at high displacement angles, and the rapid fall-off of torque at high angles can thereby cause poor control response and unpredictable steering performance.
Another type of steering actuator used in modern commercial aircraft is a rack and pinion steering assembly. Rack and pinion assemblies employ a system of involute gears and a hydraulically actuated rack to translate linear force from a hydraulic actuator to rotational torque applied to the landing gear shock strut. This type of assembly is complex, expensive and requires frequent adjustment of the gear system to control backlash and to avoid possible shimmy problems. Both rack and pinion and other types of gear-driven steering assemblies frequently suffer from such problems because virtually all of the steering load is transmitted through a single gear tooth. This problem is typically minimized, though not altogether avoided, by using large and heavy gears, an approach which is generally undesirable in resolving problems of aircraft mechanical components.
Accordingly, it is an object of the present invention to provide a nose landing gear steering actuator which does not employ any gear-driven mechanisms.
It is a further object of the present invention to provide a nose landing gear steering actuator which provides constant torque to the nose landing gear over its full range of angular displacement.
It is yet another object of the present invention to provide a nose landing gear steering actuator which can deflect the nose landing gear at least approximately 75.degree. in either direction from a dead center position.